The task involves providing a low-volatile resin additive that allows for improving refractive index with minimal addition amount. The solution includes compounds such as 1,2-bis(dinaphthothien-6-ylmethoxy)ethane, bis(dinaphthothien-6-ylmethyl)succinate, bis(dinaphthothien-6-ylmethyl)terephthalate, 1,4-bis(dinaphthothien-6-ylmethoxy)benzene, 1,2-bis(2-(dinaphthothien-6-yl)ethylthio)ethane, 1,4-bis(2-(dinaphthothien-6-yl)ethylthio)benzene, and similar compounds. No specific figures are provided.